It is well known that many organic compounds such as aromatic hydrocarbons fluoresce when irradiated with electromagnetic radiation of certain wavelengths. The property of fluorescence, particularly in response to ultra-violet radiation, had been used for many years as the basis of a method of chemical analysis. Fluorescence analysis has also been performed on cuttings or cores obtained during the drilling of wells to determine the presence of hydrocarbons in pore fluid. An example of such a technique can be found in U.S. Pat. No. 4,690,821. In techniques such as these, the cuttings or cores are cleaned to remove any drilling fluid products which might otherwise interfere with the analysis and the samples are crushed and extracted with a solvent which is then analyzed. Alternatively the sample is irradiated directly and the fluorescence analyzed. While this technique can provide reasonably accurate analysis of the pore fluids, there are certain drawbacks. Cores are relatively expensive to obtain and must be returned to the surface for analysis. Also, since cores are only taken at specific locations, it is possible that a hydrocarbon-bearing formation can be missed. Cuttings are obtained continuously in drilling, but have the disadvantage that it is not possible to determine at the surface exactly where the cuttings originate downhole making the identification of hydrocarbon-bearing formations difficult. Also the cuttings give no accurate indication of the extent of any hydrocarbon bearing formations.
It has been proposed to use fluorescence of hydrocarbons for downhole evaluation of pore fluids with a wireline logging tool. U.S. Pat. No. 2,206,922 discloses a wireline tool which is lowered into a well and which uses fluorescence in response to ultra-violet radiation to detect hydrocarbons. In one application, it is proposed to use this tool to detect the presence of oil in a drilling fluid in the well as an indication of oil-bearing strata. In another application it is proposed to bail drilling fluid from the well and to examine the borehole wall for oil. Neither application is feasible. In the first case, the presence of any fluorescence materials, such as oil, which are added to the drilling fluid-in use will mask fluorescence from oil entering the borehole from the formation. In any event, only small amounts of oil will enter a borehole due to the mud overpressure. Also, oil entering the borehole from the formation will become dispersed during drilling or will migrate up the well due to its differing density to the drilling fluid. Therefore this approach will not allow accurate identification of oil bearing strata. In the second case, the option of taking drilling fluid from the well is not usually available in well drilling and in any case this will not remove mud cake from the borehole wall which will mask the underlying formations. U.S. Pat. No. 2,206,922 fails to disclose any method or apparatus which might conceivably work in the desired manner in an oil well drilled using current technology. In particular, there is no disclosure of a technique which could be used to reliably log the length of the well with any expectation of being able to identify hydrocarbon-bearing formations.
U.S. Pat. No. 2,346,481 proposes a fluorescence logging tool which can be lowered into a well and logged to identify oil-bearing strata. In this case, the tool includes an ultra-violet light source and a transparent bar of quartz projects from the light source and is maintained in contact with the borehole wall. A further quartz bar is provided to direct the fluorescence to a film recorder as the tool is logged through the borehole. While it appears to address some of the problems associated with the approach in U.S. Pat. No. 2,206,922, the system described would still fail to function as described or intended. In order to examine the borehole wall, it would be necessary for the quartz rods to penetrate the mudcake formed thereon. However, the force which must be applied to the quartz bar to ensure that this would take place would be too high and the bars would fail mechanically. Also the action of logging the tool through the borehole would ignore forces on the quartz bars which would cause them to fail even if the mudcake were not significant. Therefore the approach set forth in U.S. Pat. No. 2,346,481 is also inoperable.
U.S. Pat. No. 2,334,475 also discloses a fluorescence logging tool for detecting petroleum in situ. The tool includes an ultraviolet source which illuminates the rock through a quartz window and detects fluorescence through the same window. The window is suggested as being glass or quartz and, in one case, is housed in a mounting which includes portions for cutting through the mudcake on the borehole wall. It is suggested that quartz be selected as a material which is sufficiently resistant to withstand the friction of the borehole wall during the logging application and that the window project from its housing to come into direct contact with the borehole wall. However quartz is not sufficiently strong to withstand the forces generated during logging and will fail by fracturing. Thus the embodiment described in U.S. Pat. No. 2,334,475 also is inoperable and in all likelihood would not withstand more than a few feet of logging before tool failure.
It is an object of the present invention to provide a method and apparatus for identifying in situ hydrocarbons using fluorescence.